Kinins belong to a family of bioactive octa- to decapeptides generated from the inactive precursor kininogen in a stepwise proteolytic process in body fluids and tissues. Kinins are hormones formed by a group of 9-11 amino acid peptides, including bradykinin (BK), kallidin (KD/Lys-BK), and their active metabolites (des-Arg9-BK and des-Arg10-kallidin/Lys-des-Arg9-BK). The kinins play an important physiological role in inflammatory and nociceptive processes. The biological effects of BK and other kinins are mediated by two physiologically distinct G protein-couple receptors (GPCRs), termed BDKRB1 (B1R) and BDKRB2 (B2R). It is believed that under physiological conditions, the constitutively expressed B2R mediates the effects of circulating or locally generated kinins, since B1R is not expressed in normal tissues. The B2R is constitutively expressed in numerous cell types of the central and peripheral nervous systems, the vascular endothelium and inflammatory cells, and is activated by the short lived natural ligands, BK and kallidin (KD). Once synthesized, BK causes vasodilatation and increased vascular permeability by interaction with B2Rs. However, B2Rs are rapidly desensitized and internalized following binding and activation by the endogenous ligands. Catalytic degradation of kinins by enzymes including carboxypeptidase N and carboxypeptidase M yields des-Arg9-BK (DABK) and des-Arg10-kallidin/Lys-des-Arg9-BK, which preferentially activate the B1R. Although not expressed in normal tissues (or expressed at very low levels), the B1R is rapidly induced following bacterial infections, tissue injury and release of inflammatory mediators, and has been observed in sympathetic neurons, macrophages, fibroblasts, smooth muscle cells, and the vascular endothelium. The endogenous B1R agonists, including des-Arg9-BK and des-Arg10-kallidin/Lys-des-Arg9-BK, are relatively long-lasting peptides. Moreover, the B1R does not undergo rapid desensitization and internalization after stimulation. Once upregulated, B1R activity persists in damaged or inflamed tissues and is thought to participate in prolonging the pathological response to kinins. Thus, the B1R has been implicated in maintaining chronic pain, vasodilation, plasma extravasation, neutrophil recruitment and further release of inflammatory mediators, such as IL-β, TNF-α and IL-6 which sustain a positive feedback loop between B1R expression and inflammation. The proposed upregulation of B1R only under pathological conditions, including inflammation, trauma, burns, shock, and allergy, makes B1R a particularly attractive drug target.
The proposed role of kinins in mediating pain and inflammation has propelled interest in the discovery of potent and selective BK antagonists. Recent evidence suggests that bradykinin receptors may also play an important role in a number of pathological processes or diseases, including ischemia-reperfusion injury, diabetic retinopathy, atherosclerosis, renal disease. Hence, there is an urgent need for novel compounds that are effective in blocking or reversing activation of bradykinin receptors. Such compounds would be useful in the management of pain and inflammation, as well as in the treatment or prevention of diseases and disorders mediated by bradykinin; further, such compounds are also useful as research tools.